Ich drehe mich um, wenn meine Mutter meinen Namen ruft.

Because umdrehen is a separable verb.

In a normal main clause, the conjugated part goes to the usual verb position, and the prefix moves to the end:

• infinitive: sich umdrehen
• main clause: Ich drehe mich um.

So:

• drehe = the conjugated verb
• um = the separated prefix

This is very common in German:

• Ich stehe auf.
• Er ruft an.
• Wir gehen aus.

With a modal or another infinitive construction, it stays together:

• Ich will mich umdrehen.

In a subordinate clause, it also stays together at the end:

• ..., wenn ich mich umdrehe.

Because sich umdrehen is usually used reflexively when someone turns themselves around.

German often says this as to turn oneself around, even where English usually just says I turn around.

So:

• ich → mich
• du → dich
• er/sie/es → sich

Examples:

• Ich drehe mich um.
• Du drehst dich um.
• Sie dreht sich um.

Here mich is the reflexive pronoun that matches ich.

Because in a normal German statement, the finite verb has to be in second position.

Since umdrehen is separable:

• the conjugated part goes in position 2: drehe
• the prefix goes to the end: um

So the correct word order is:

• Ich drehe mich um.

Not:

• Ich umdrehe mich.

German word order is much stricter than English here.

Because wenn introduces a subordinate clause, and in German subordinate clauses are normally separated by a comma.

So the sentence is divided into:

• main clause: Ich drehe mich um
• subordinate clause: wenn meine Mutter meinen Namen ruft

That is why you get:

• Ich drehe mich um, wenn meine Mutter meinen Namen ruft.

This comma is required in standard German.

Because the part starting with wenn is a subordinate clause, and in German subordinate clauses the conjugated verb usually goes to the end.

So:

• wenn = subordinating conjunction
• subject: meine Mutter
• object: meinen Namen
• verb at the end: ruft

That gives:

• wenn meine Mutter meinen Namen ruft

This is one of the most important German word-order rules.

Compare:

• main clause: Meine Mutter ruft meinen Namen.
• subordinate clause: ..., wenn meine Mutter meinen Namen ruft.

Wenn is the right choice here because it means when in the sense of whenever or if/when.

This sentence describes something that happens generally or repeatedly:

• I turn around when my mother calls my name
• in other words, whenever my mother calls my name

Compare:

• wenn = when/whenever, if
• als = when, but only for a single event in the past
• wann = when? in a question or indirect question

So:

• Wenn meine Mutter meinen Namen ruft, drehe ich mich um. = correct
• Als meine Mutter meinen Namen rief, drehte ich mich um. = one specific past event
• Wann ruft meine Mutter meinen Namen? = question

Because they are in different cases.

• meine Mutter is the subject, so it is in the nominative
• meinen Namen is the direct object, so it is in the accusative

That is why the possessive word changes:

• nominative masculine: mein
• accusative masculine: meinen

But Mutter is feminine, so in nominative it is:

• meine Mutter

And Name is masculine, so in accusative it becomes:

• meinen Namen

So the forms are showing the grammatical role of each noun.

Because Name is a masculine noun of a special type often called an n-noun or weak masculine noun.

These nouns add -n or -en in most cases except the nominative singular.

So:

• nominative: der Name
• accusative: den Namen
• dative: dem Namen
• genitive: des Namens

In your sentence, Namen is accusative singular, so the -n appears:

• meinen Namen

Other common nouns like this are:

• der Junge → den Jungen
• der Mensch → den Menschen
• der Student → den Studenten

Yes. Rufen can take a direct object, and one common pattern is:

• jemandes Namen rufen = to call someone’s name

So:

• meine Mutter ruft meinen Namen

is a normal German structure.

German can also say:

• Meine Mutter ruft mich.

But that sounds more like my mother is calling me rather than specifically calling my name.

So meinen Namen ruft is appropriate if the sentence is specifically about hearing your name being called.

Yes. Both verbs are in the present tense:

• drehe
• ruft

In this sentence, the present tense expresses a general/habitual situation:

• whenever this happens, I turn around

German uses the present tense for this very naturally, just like English often does:

• I turn around when my mother calls my name.

It is not necessarily happening right now; it can describe a regular pattern.

Yes. German very often allows that:

• Wenn meine Mutter meinen Namen ruft, drehe ich mich um.

This means the same thing.

But notice what happens in the second clause: the finite verb still has to come in the normal V2 pattern, so after the whole wenn clause, German puts the verb immediately:

• Wenn meine Mutter meinen Namen ruft, drehe ich mich um.

Not:

• Wenn meine Mutter meinen Namen ruft, ich drehe mich um.

That is another very important word-order rule in German.